Physiological and molecular responses of seedlings of an upland rice ('Tung Lu 3') to total submergence compared to those of a submergence-tolerant lowland rice ('FR13A').

Background: Understanding the responses of rice to environmental stresses such as unscheduled submergence is of pressing important owing to increasing severity of weather thought to arise from global climate change. When rice is completely submerged, different types adopt either a quiescence survival strategy (i.e., minimal shoot elongation) or an escape strategy (i.e., enhanced shoot elongation). Each strategy can prolong survival depending on the circumstances. While submergence responses have been studied in rice typical of lowland and flood-prone areas, few studies have explored the physiological and molecular properties of upland rice under submergence. Here, we use seedlings of the upland rice 'Tung Lu 3' ('TL3') to analyze physiological and molecular responses to submergence. We compare them with those of 'FR13A', a lowland rice that tolerates submergence by adopting the quiescence strategy.

Results: Plant height and distance between leaf sheaths, increased rapidly in 'TL3' under submergence. Although this indicated a strong escape strategy the seedlings remained totally underwater for the duration of the experiments. In contrast, 'FR13A' elongated much less. Consequently, after 4 days complete submergence followed by drainage, 'TL3' lodged much more severely than 'FR13A'. After 10 d complete submergence, 55% of 'TL3' seedlings survived compared to 100% in 'FR13A'. Chlorophyll a, b and total chlorophyll concentrations of the 2 oldest leaves of 'TL3' were also significantly above those of 'FR13A' (but were lower than 'FR13A' in the 3 oldest leaves) and less hydrogen peroxide accumulated in 'TL3'. Peroxidase activity in submerged 'TL3' was also greater than in 'FR13A' 1 day after submergence. Quantitative RT-PCR showed increased expression of sucrose synthase 1 and alcohol dehydrogenases 1 after 2 days complete submergence with significantly higher levels in 'TL3' compared to 'FR13A'. Expression was also higher in 'TL3' under non-submerged conditions.

Conclusions: The upland rice line 'TL3' gave a stronger elongation response than 'FR13A' to complete submergence. This escape strategy is widely considered to prejudice survival when the plant remains totally submerged. However, contrary to expectations, 'TL3' survival rates were substantial although below those for 'FR13A' while physiological, biochemical and molecular parameters linked to adaptation differed in detail but appeared to be broadly comparable. These findings highlight that submergence tolerance is determine not only by the adoption of quiescence or escape strategies but maybe by metabolic and physiological properties unrelated to the underwater elongation rate.